Probing Into Microscopic Features Underlying T_A Of Metallic Glass-forming Liquids

The microstructural and dynamic evolutions of metallic glass-forming liquids（MGFLs）upon cooling have always been the hot issues in materials science and condensed matter physics.Recent studies show that the relation between relaxation time and temperature experiences a crossover from Arrhenius to non-Arrhenius at a high temperature,T_A,during cooling.Furthermore,the ratio of T_A and T_g is about 2,which suggests that the features related to the glass transition are already present from T_A in the melt.A deep insight into the microscopic features underlying T_A is not only of great significance for revealing the nature of glass transition,but also instructive for controlling and improving the performance of MG solids from the aspect of liquids.Given that researches on the microscopic features underlying T_A has just been started recently,we should put more efforts into the systematic works in this field.In this dissertation,we investigated the important atomic-scale structural and dynamic features for different MGFLs especially near T_A by using molecular dynamics（MD）simulations.The main content is as follows:The general structural features near non-Arrhenius crossover temperature,T_A,was investigated at atomic level in Fe₈₀P₂₀,Fe₈₀Ni₂₀ and Cu₅₀Zr₅₀ MGFLs.Notable changes have been found in the connectivity behavior of the characteristic atoms with higher local five-fold symmetry（LFFS）near T_A.The characteristic atoms in the melt above T_A are weakly correlated and the atomic dynamics is relatively homogeneous.As the temperature decreases to T_A,the characteristic atoms aggregate in a denser way,beginning to form stable three-dimensional networks and much bigger clusters.The atomic correlation in the liquid is significantly enhanced and the free motion of atoms is limited,leading to cooperative motion.Consequently,the dynamic heterogeneity also displays a prominent increase deviating from Arrhenius near T_A.This work uncovers the microscopic landscape corresponding to the non-Arrhenius crossover of macroscopic relaxation behavior in MGFLs,which is crucial to understand the nature of glass transition from the aspect of high-temperature liquid.Based on the above work.we further focus on the dynamic properties from high-temperature melt to near glass transition for 12 different MG systems.By introducing the non-Arrhenius crossover temperature,T_A,and the corresponding structural relaxation time,τA,a scaling collapse for the temperature dependence of the relaxation time（and dynamic heterogeneity）is obtained.The general relation between relaxation time and dynamic heterogeneity is also established.We reveal that the dynamic heterogeneity at T_A is constant in different MG systems,corresponding to a2.max≈ 0.2.Besides,the characteristic temperature T*（and characteristic time τ*）obtained through identical dynamic heterogeneity condition has a linear relation with T_A（and τA,）.This work provides a method to predict the dynamic properties of MGFLs from the high-temperature liquid:Once both T_A and rA are measured,the relaxation time during the whole cooling process can be predicted;if we only know T_A or τA,the evolution of dynamic heterogeneity can also be predictedWith deeper insights into the microscopic evolution of MGFLs,establishing the correlation between liquid and solid has become the focus in actual production.The general atom-level features related to glass-forming ability（GFA）in Fe-based liquids are investigated by the addition of large metallic（Ni）and small non-metallic（P）elements,respectively,into pure Fe system.Simulation results demonstrate the GFA of both the Fe₈₀Ni₂₀ and Fe₂₀P₂₀ liquids is better than that of pure Fe.In both Fe₈₀Ni₂₀ and Fe₂₀P₂₀ glass-forming liquids,more icosahedron-like clusters（<0,0,12,0>,<0.1,10,3>and<0,1,10,2>）with slow dynamics form rapidly as the temperature decreases to below T_A.accompanied with depressing bcc-like clusters（<0,3,6,4>,<0,3,6,5>.<0.2,8.4>and<0.,4.4,6>）with faster dynamics.This suggests a competition between them and icosahedron-like clusters are beneficial to the glass transition in Fe₈₀Ni₂₀and Fe₂₀P₂₀.Also.the dynamic heterogeneity of Fe₈₀Ni₂₀ and Fe₈₀P₂₀ supercooled liquids experiences a more dramatic increase before glass transition.These findings reveal the general microstructural and dynamic characteristics in Fe-based MGFLs beneficial to GFA and shed light on how to enhance GFA from the the aspect of liquid.